A kinetic study of mass transfer in reversed-phase liquid chromatography on a C18-silica gel

The characteristic features of mass-transfer kinetics in a reversed-phase (RP) column packed with a C18-silica were studied. The relevant information on phase equilibrium thermodynamics and mass-transfer kinetics was obtained by frontal analysis and the pulse method, respectively. The equilibrium isotherm was accounted for by the simple Langmuir model. The ratio of the axial dispersion coefficient to the mobile-phase flow velocity increased almost linearly with increasing solute concentration. Similarly, the mass-transfer rate coefficient (km) showed a linear dependence on the solute concentration. The positive concentration dependence of km resulted from that of the surface diffusion coefficient, which was interpreted with the chemical potential driving force model. The contribution of axial dispersion to band broadening was predominant in the RP column packed with the medium-size packing material used (particle diameter, 12 microns) whereas that of the kinetics of adsorption/desorption was negligibly small. The results of this study demonstrate how an analysis of the dependence of the mass-transfer kinetics on the flow velocity and the solute concentration allows a better understanding of this kinetics.